1. Field of the Invention
The present invention relates to an image forming apparatus such as an electrophotographic copying machine, and an electrophotographic printer and the like, and a process cartridge.
2. Related Background Art
Conventionally, as electrophotography, there has been proposed a method in which an electrical latent image is formed on a photosensitive body having a photo-conductive surface in many ways, the latent image is developed with toner as a visualized image or toner image, the toner image is transferred onto a transfer material such as paper on demand, and, then, the toner image is fixed to the transfer material by heat and pressure, thereby obtaining a copy.
Nowadays, since improvement in resolving power and sharpness of an image have been requested, formation of a thin toner layer and associated devices must be developed more and more, and various methods for achieving this have been proposed.
For example, recently, there has been proposed a developing method of a non-magnetic one-component DC contacting type in which a semi-conductive developing roller or a developing roller having a dielectric surface layer is used and development is effected by contacting such a developing roller with a surface of a photosensitive body.
FIG. 10 shows such a developing method of a non magnetic one-component DC contacting type. In this method, a photosensitive drum 21 and a developing roller 81 contact each other and are rotated in the directions shown by the arrows A, B, respectively. The photosensitive drum 21 is charged by a charging roller 2, and a latent image is formed on the photosensitive drum 21 by a laser beam from an exposing unit 5. The latent image is visualized by a developing apparatus 80. Thereafter, toner 84 on the photosensitive drum 21 that is developed is transferred onto a transfer material 9 by a transfer roller 6 and then the toner image is fixed to the transfer material by a fixing apparatus 7. On the other hand, residual toner 84 remaining on the photosensitive drum 21 that was not transferred is removed by a cleaning blade 3 and then is collected into a cleaning container 4.
In the developing apparatus 80, there is provided an elastic roller 82 disposed within a developing container 85 containing toner 84 as one-component developer and urged against the developing roller 81 at an upstream side of an elastic blade 83 in a rotational direction of the developing roller 81, and, by rotating tho elastic roller in a direction shown by the arrow D, the toner 84 is supplied onto the developing roller 81. The toner 84 supplied to the developing roller 81 is carried as the developing roller 81 is rotated, and then the toner is charged by friction in an abut area between the elastic blade 83 and the developing roller 81, thereby forming a thin layer. The thin toner layer 84 is shifted by the developing roller 81 and is supplied to the electrostatic latent image in a nip between the developing roller and the photosensitive drum 21. Thereafter, the toner 84 remaining on the photosensitive drum (which was not applied to development in the nip between the photosensitive drum 21 and the developing roller 81) is removed by the elastic roller 82, and at the same time, as mentioned above, new toner 84 is supplied onto the developing roller 81. Such operations are repeated.
However, the above-mentioned conventional developing method of a non-magnetic one-component DC contacting type has the following problem.
FIG. 11 shows an image gradient of 64 gradations in 600 dpi in the above-mentioned method. The conventional photosensitive drum 21 is an organic photosensitive body of a laminated type and includes a charge transport layer having a thickness of 25 ,um. As can be seen from FIG. 11, in this method in which the development is effected by using DC voltage, density can quickly rise, but, image density quickly enters a saturated condition, and, thus, even when the gradient is improved, an image having a substantially high gradient may not be obtained.
To avoid this, if the charging amount of toner is increased, although the image density can moderately be increased, in the conventional example shown in FIG. 11, an optimum range in which the saturation of density and the desired gradient become compatible is only xe2x88x9230 xcexcC/g, which is vers narrow.
An object of the present invention is to provide an image forming apparatus and a process cartridge in which an optimum range of the toner charging amount is wide and an excellent gradient can be obtained.